In many industries, like the petrochemical and chemical industries, contact of reaction fluids with a catalyst in a reactor under suitable temperature and pressure conditions effects a reaction between the components of one or more reactants in the fluids. Most of these reactions generate or absorb heat to various extents and are, therefore, exothermic or endothermic. The heating or chilling effects associated with exothermic or endothermic reactions can positively or negatively affect the operation of the reaction zone. The negative effects can include among other things: poor product production, deactivation of the catalyst, production of unwanted by-products and, in extreme cases, damage to the reaction vessel and associated piping. More typically, the undesired effects associated with temperature changes will reduce the selectivity or yield of products from the reaction zone.
Processes for the production of hydrogen and carbon oxides by reforming methane in the presence of steam or carbon oxides have been practiced for many years. The steam reforming process is particularly well known, and involves passage of a mixture of feedstock and steam over a steam reforming catalyst. Typical steam reforming catalyst comprises nickel and may include cobalt on refractory supports such as alpha alumina or calcium aluminate. The strong endothermic nature of the primary steam reforming reaction requires a supply of heat to maintain the reaction. Those skilled in the art routinely balance the endothermic heat requirements of the primary reforming reaction with a partial oxidation of hydrocarbons to provide a secondary reforming reaction that supplies heat for the primary reforming stage and generates additional synthesis gas. Extensive and highly developed teachings detail methods of indirectly exchanging heat between primary and secondary reforming zones. The operation of an adiabatic reformer for synthesis gas production is shown in U.S. Pat. Nos. 4,985,231. 5,300,275 sets forth another basic arrangement that uses a secondary reforming reaction to supply hot gas for heating the primary reforming reaction. U.S. Pat. Nos. 4,810,472; 4,442,020; 4,750,986; and 4,822,521 disclose particular arrangements of heat exchange reactors that indirectly exchange heat between hot gases from the secondary reforming stage and the primary reforming stage. U.S. Pat. No. 4,127,389 shows a variety of tube chamber designs for supplying heat to a primary reforming reaction from a secondary reforming reaction zone. As established by the above referenced patents, the art currently relies exclusively on tube arrangements, and most commonly relies on double walled tubes referred to as "bayonet tubes, for exchanging heat between the primary and secondary reforming zones. The geometry of tubular reactors poses layout constraints that require large reactors and a vast tube surface to achieve high heat transfer efficiencies.
Other process applications accomplish indirect heat exchange with thin plates that define channels. The channels alternately retain catalyst and reactants in one set of channels and a heat transfer fluid in adjacent channels for indirectly heating or cooling the reactants and catalysts. Heat exchange plates in these indirect heat exchange reactors can be flat or curved and may have surface variations such as corrugations to increase heat transfer between the heat transfer fluids and the reactants and catalysts. Many hydrocarbon conversion processes will operate more advantageously by maintaining a temperature profile that differs from that created by the heat of reaction. In many reactions, the most beneficial temperature profile will be obtained by maintaining substantially isothermal conditions. In some cases, a temperature profile directionally opposite to the temperature changes associated with the heat of reaction will provide the most beneficial conditions. For such reasons it is generally known to contact reactants with a heat exchange medium in cross flow, cocurrent flow, or countercurrent flow arrangements. A specific arrangement for heat transfer and reactant channels that offers more complete temperature control can be found in U.S. Pat. No. 5,525,311; the contents of which are hereby incorporated by reference. Other useful plate arrangements for indirect heat transfer are disclosed in U.S. Pat. Nos. 5,130,106 and 5,405,586.
Long sought objectives of reforming processes for the production of synthesis gas are the provision of highly efficient heat exchange and a reduction of fuel requirements which in turn serve to raise product yields from the process. Efficient heat exchange with the secondary reforming zone can provide a potentially auto thermal process. Conversion of hydrocarbons from the feed or primary reforming zone effluent in the secondary reforming zone presents an additional yield loss to the extent that oxidation produces heat rather than desired products. In particular, large quantities of hydrogen or an ammonia synthesis mixture of hydrogen and nitrogen can be produced by steam reforming operations or by the partial oxidation reactions. The synthesis gas streams are typically produced for use as feedstocks in downstream processing such as the production of methanol, formaldehyde, or dimethyl ether.
Variations in the reactor operations, the composition of the feed, and amount of the feed sent to the primary versus the secondary reforming zone can be used to control the H.sub.2 :CO ratios Ts generated in the reformer effluent. Adjusting the H.sub.2 :CO to suit the stoichiometry requirements of downstream processing improves process integration. The principal shift reaction of methane and steam to CO and H.sub.2 in the primary reforming reaction produces a 3:1 H.sub.2 :CO ratio. Partial oxidation of methane in the secondary reaction produces a 2:1 H.sub.2 :CO ratio. Over oxidation converts H.sub.2 and CO to undesired water and CO.sub.2. Dependence on the secondary reforming reaction to produce heat restricts variations in the feeds and operating conditions, limits the range of H.sub.2 :CO ratios that may be obtained in the synthesis gas effluent, and interferes with the supply of a synthesis gas to a downstream processing reaction with the desired stoichiometric H.sub.2 :CO ratio. U.S. Pat. Nos. 4,910,228 and 5,512,599 are particularly directed to achieving a desired heat integration to provide a secondary reforming gas having the approximate stoichiometric requirement of a feed for downstream methanol production.
The production of synthesis gas may find particular utility in the supply of feed to produce methanol or higher hydrocarbons from methane by methods such as the Fisher Tropsch process. Facilitating the production of hydrocarbon synthesis feeds can promote the utilization of the large proportion of natural gas that typically accompanies the discovery of petroleum reserves. Most of these reserves are discovered in remote areas where transportation of low molecular weight gas proves uneconomical. Accordingly, there is a need for compact and efficient equipment that is easily transported to, and operated in, the isolated oil fields where the natural gas is found.
It is, therefore, an object of this invention to improve the efficiency of indirect heat transfer from a secondary reforming reaction to a primary reforming reaction.
It is a further object of this invention to reduce the combustion requirements of synthesis gas feeds for supplying heat to a primary reforming reaction.
It is a further object of this invention to reduce the fuel requirements for supplying heat to a primary reforming reaction from a secondary reforming reaction to provide greater control over the ratio of components in synthesis gas product.
It is a yet further object of this invention to provide a greater range of stoichiometric ratios for feedstocks derived from synthesis gas production.
An additional object of this invention is to provide a compact equipment arrangement for the production of syngas from natural gas.